1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of Related Art
An H-bridge circuit is generally used as a circuit for driving a motor. If a motor is driven by the H-bridge circuit, a Zener diode is used to constrain voltage increase so as not to cause breakdown of MOSFET due to the voltage increase when a kickback is generated (e.g., Japanese Patent Application Laid-Open Publication No. 2005-269885).
FIG. 4 is a diagram of a configuration example of a motor drive circuit that uses a Zener diode to constrain voltage increase. A current source 130, NPN-transistors 131 to 133, and a resistor 134 are disposed for a circuit controlling ON/OFF of a P-channel MOSFET 101.
If the NPN-transistor 131 is turned off, a current output from the current source 130 flows into the NPN-transistor 132, and a current also flows through the NPN-transistor 133 connected in a current mirror in accordance with a mirror ratio. As a result, a current also flows through the resistor 134; a voltage obtained by reducing a voltage Vm of a power line 112 is applied to the gate of the MOSFET 101; and the P-channel MOSFET 101 is turned on.
On the other hand, if the NPN-transistor 131 is turned on, the current output from the current source 130 flows into the NPN-transistor 131, and therefore, no current flows through the resistor 134. As a result, the gate voltage of the P-channel MOSFET 101 is raised to the voltage Vm of the power line 112, and the P-channel MOSFET 101 is turned off.
An NPN-transistor 140 and a PNP-transistor 141 are disposed for a circuit controlling ON/OFF of an N-channel MOSFET 103. If the NPN-transistor 140 is turned on and the PNP-transistor 141 is turned off, the gate voltage of the N-channel MOSFET 103 is raised to the voltage Vm of the power line 112 and the N-channel MOSFET 103 is turned on.
On the other hand, if the NPN-transistor 140 is turned off and the PNP-transistor 141 is turned on, the gate voltage of the N-channel MOSFET 103 is reduced to a voltage of a ground line 113 and the N-channel MOSFET 103 is turned off.
Similarly, a current source 150, NPN-transistors 151 to 153, and a resistor 154 are disposed for a circuit controlling ON/OFF of a P-channel MOSFET 102. An NPN-transistor 160 and a PNP-transistor 161 are disposed for a circuit controlling ON/OFF of an N-channel MOSFET 104. A drive circuit 170 controlling the drive of the motor by controlling ON/OFF of the transistors is implemented.
If the P-channel MOSFET 101 and the N-channel MOSFET 104 are turned on and the P-channel MOSFET 102 and the N-channel MOSFET 103 are turned off in such a motor drive circuit, a current flows from the power line 112 toward the P-channel MOSFET 101, a motor coil 105, and the N-channel MOSFET 104, and the motor rotates in one direction. If the P-channel MOSFET 101 and the N-channel MOSFET 104 are turned off from this state at any timing, the current tries to continue flowing because of energy accumulated in the coil 105. Therefore, a kickback current is generated via a parasitic diode 103d of the N-channel MOSFET 103 and a parasitic diode 102d of the P-channel MOSFET 102.
This kickback current cannot be regenerated on the power source 110 because of a diode 114 and flows into a condenser 121. Therefore, the voltage Vm of the power line 112 is increased. Even when the diode 114 does not exist, the voltage Vm of the power line 112 is increased in such a case that the power line 112 is long. In this situation, the increase in the voltage Vm of the power line 112 is constrained by a Zener diode 120 to prevent the P-channel MOSFET 101 and 102 from being broken.
By the way, as a motor size is enlarged, the size of the Zener diode 120 must be enlarged since the increase in the voltage Vm of the power line 112 becomes larger. The size of the condenser 121 must also be enlarged to about 100 μF to 1000 μF. The enlargement of the sizes of the Zener diode 120 and condenser 121 cause cost increase.